1. Field of the Invention
The invention relates to the field of vector plotters and, in particular, to an ink sensing system for a vector plotter in which an optical detection system determines the degree of reflectance from a drawing media surface to indicate the presence or absence of ink on the media surface.
2. Description of Related Art
In the field of plotters using ink pens, it is necessary to determine, continuously or at intervals, that the ink is being properly laid down on the drawing media surface during the plotting process. Such failure can be due to lack of ink in the pen or due to obstructions in the pen itself which prevents the ink from flowing freely onto the media surface. In the former instance, infrared ink-level sensing systems have been proposed to monitor the ink level in the pen itself. In the latter instance, sensing systems have been proposed, such as those described in U.S. Pat. Nos. 3,335,287 and 4,435,674, which utilize light-emitting diodes and photodiode detectors carried on the drawing head to continuously monitor the line being drawn by detecting variations in the intensity of light reflected from the drawing media surface due to the presence or absence of ink to absorb the light focussed on the surface. These sensing systems have, however, several disadvantages. Since the sensors comprise a circular array of optical fibers located around the scriber, the optical fibers focus light on and scan an annular area of the plotting paper surrounding the tip of the pen. Since only a small portion of the focused light can be absorbed by ink on the surface of the media, the sensitivity to small variations in detected light is critical to the successful operation of the system. Furthermore, since the detector system looks at an annular area around the tip of the pen, the system is sensitive to what has been drawn but cannot detect the current status of the ink flowing onto the media surface. Finally, since the systems look at an annular area surrounding the pen and operate in a continuous back-looking mode, excessive power and expensive and complicated electronics are needed for successful operation of such systems.
Thus, it is a primary object of the present invention to provide an improved ink sensing system for a vector plotter.
It is another object of the present invention to provide an improved ink sensing system in which a positive indication is given as to the presence or absence of ink on a drawing media surface.
It is a further object of the present invention to provide an improved ink sensing system which can determine the current status of ink flowing onto the drawing media surface.
It is still another object of the present invention to provide an improved ink sensing system in which optical and electronic power requirements and complexity are minimized.